Added

• PMM-3401: Added collection of TTL metrics #127, thx @fastest963
• Added some new metrics:
  • member_replication_lag
  • member_operational_lag
  • op_latencies_latency_total
  • op_latencies_ops_total
  • op_latencies_histogram

Fixed

• Fix for broken labels on index_usage metrics
• Fix SIGSEGV when running connected to a Primary, thx @anas-aso

Changed

• Move CHANGELOG.md to "Keep a Changelog" format.